A backlight is used as a light source for displaying an image on a liquid crystal display panel (hereinafter referred to as LCD panel), such as a liquid crystal television, a liquid crystal display, a liquid crystal monitor, or the like. In order to supply light over the entire LCD panel, a backlight is required to have a uniform in-plane brightness distribution.
Such a backlight employs fluorescent lamps such as cold cathode fluorescent lamps (hereinafter referred to as CCFL devices), or LEDs.
LEDs are classified into a plurality of ranks regarding varied brightnesses (brightness ranks). Even LEDs classified into the same brightness rank are allowed to have variations in brightness in the range of predetermined values (brightness variations). Thus, when a backlight employing LEDs is manufactured, it is necessary to consider such variations in brightness among LEDs to be prepared so that a uniform in-plane brightness distribution can be maintained.
It should be noted that, in a backlight, an in-plane brightness distribution is absorbed to some extent because illuminating light is generally viewed through a diffusion sheet and an LCD panel. This eliminates the need to consider such variations in brightness of the same brightness rank, in the case of a backlight.
A Japanese Unexamined Patent Publication No. 105082/1998 (Tokukaihei 10-105082, publication date: Apr. 24, 1998) discloses a method for preparing LEDs having brightnesses being varied in the same brightness rank, and mounting the LEDs on the substrate so that a uniform brightness is viewed. This publication describes an LED dot matrix panel for displaying arbitrary characters and/or figures, not a backlight in which light is viewed through a diffusion sheet in the foregoing manner. Thus, even variations in brightness of the same brightness rank is considered in this publication.
As described above, in a backlight, an in-plane brightness distribution is absorbed to some extent because illuminating light passes through a diffusion sheet and an LCD panel. This eliminates the need to consider variations in brightness of the same brightness rank, unlike the case of the LED dot matrix panel described in the above publication.
That is, variations in brightness would not be a big issue if a backlight is manufactured by employing LEDs of the same brightness rank. However, manufacturing all backlights employing LEDs of the same brightness rank would cause a cost increase due to the following reasons.
For example, assume that 250 LEDs are mounted on each substrate constituting a backlight and 50,000 backlights are manufactured per month. In this case, 12,500,000 LEDs are required every month. With actual capability of manufacturing LEDs, however, it is difficult to manufacture 12,500,000 LEDs of only the same rank due to variations caused during manufacturing, even if it would be possible to manufacture 12,500,000 LEDs of plural brightness ranks. Further, if LEDs of only one brightness rank are manufactured, eventually, LEDs have to be discarded which have a brightness not falling within a desired rank. This causes a high increase in cost for purchasing and preparing only LEDs of the same brightness rank.